Method for separating a fraction of benzene hexachloride rich in the gamma isomer



aiented Sept. 4, 1951 METHOD FOR SEPARATING A FRACTION OF BENZENE HEXACHLORIDE RICH IN THE GAMMA ISOMEB James M. Scovic, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Application October 22, 1947, Serial No. 781,480

2 Claims. (01. 260-648) This invention relates to a treatment of hexachlorocyclohexane, also known as benzene hexachloride, CeHaCls (and which must not be confused with hexachlorobenzene, CsCls) to improve the color and odor and to provide fractions greatly enriched with respect to the gamma isomer.

The insecticidal activity of benzene hexachloride has been widely publicized in recent years. It is known that this activity is centered principally in the gamma isomer, and that the alpha, beta, delta and epsilon isomers have relatively little insecticidal efiect. The usual methods of preparation of benzene hexachloride inevitably produce mixtures of the alpha, beta and gamma isomers together with one or both of the delta and epsilon isomers, and possibly others as yet unisolated. As first prepared, the alpha isomer is present in the greatest proportion while the amount of the beta isomer is commonly from to per cent and that of the gamma isomer from 2 to 12 per cent. The mixed product is not only less eifective as an insecticidal agent than is the gamma isomer alone, but it usually has a strong odor which may be quite disagreeable. That odor has been found to be absent from or at a minimum in the gamma. isomer alone, and, regardless of whether the odor is due to one or a mixture of the other isomers or is due to common impurities, it is deemed desirable to remove it from those fractions of the product which are to be used for household insecticidal purposes.

Benzene hexachloride is most/commonly produced by the addition of chlorine to benzene. For some purposes the benzene may be diluted with other organic liquids. In either event, the chlorination is usually continued until the liquid medium becomes nearly saturated with benzene hexachloride, or until the benzene hexachloride first begins to crystallize therefrom. Various modes of recovery have been reported. If the benzene-containing organic solvent is merelyevaporated or boiled from the benzene hexachloride, either under vacuum or at atmospheric pressure, the benzene hexachloride is a dark material with a pungent odor, which has been mentioned above. If the benzene-containing organic solvent is steam distilled from the benzene hexachloride until a mush of crystals and oil is obtained, and the latter is chilled as by mixing with cold water, the product still has the irritating odor, but is considerably improved in color. It has the same ratio of isomeric forms of benzene hexachloride as has the crude material originally dissolved in the benzene, and as the product which can be obtained by boiling the original reaction mixture to dryness. It is known that the various isomers of benzene hexachloride present in any of the suggested mixed products can be separated from one another with varying degrees of completeness by means of fractional crystallization, but such a process is complicated, and too expensive for commercial use in the preparation of economic insecticides. It would be preferable to employ a method as simple as steam distillation to provide the improvement in color it such method could also provide a practical separation of the crude mixture into portions, one of which contains most of the gamma isomer produced in the original chlorination.

It is an object of the present invention to provide a process for the practical separation of a mixture of isomeric forms of benzene hexachloride into portions, on of which contains most of the gamma isomer present in the original mixture. A related object is to provide such a process from which the isolated products have a low odor. level and a negligible amount of discoloration. A further object is to base such a process on a steam distillation of an organic solution of benzene hexachloride. Related objects may appear hereinafter.

In accordance with the present invention, a solution of benzene hexachloride in an organic solvent, suitably .the solution resulting from the addition chlorination of benzene in known manner, is mixed with water and subjected to steam distillation only until from about 40 to about 80 per cent, and preferably from to per cent, of the originally dissolved benzene hexachloride has separated as a dispersion of crystals in the water layer, filtering the mixture of water. crystals and remaining solution of benzene hexachloride in organic liquid, to separate the crystals from the rest, removing residual solvent from the oily layer which passes through the filter, and cooling the filtrate to separate therefrom the rest of the benzene hexachloride. The first crop of crystals which separates during steam distillation has much less of the gamma isomer than While some difllculty is to be expected in determining when any specific proportion of the original benzene hexachloride mixture has been precipitated during steam distillation, a reasonably close estimate can be made after a little experience has been gained with the specific equipment employed, especially when a standard load is charged to the steam still. The advantages of the described process are obtained whenever the first batch of crystals separated from the process represents from about to 80 per cent of the original charge. When too few of the crystals are precipitated before the separation, both fractions retain too much of the undesired odor. This is minimized when steam distillation is continued until precipitation has reached about 40 per cent or more.

The method may be applied to solutions of benzene hexachloride in any solvent or mixture of solvents capable of being steam distilled. The

amount of solvent steam distilled from the mixture, relative to that originally present, may be used as a rough gauge of the extent of precipitation in the steam still. Starting with a normal mixture of isomers in a solution which is nearly saturated with respect to benzene hexachloride, it has been found that when the amount of distilled solvent is between and 99.5 per cent of' that originally present, the amount of crystals precipitated in the steam still is generally in the desired range of about 40 to per cent of the benzene hexachloride present. These limits, again, are not precise, since there is variation encountered with changes in the solvent mixture and for different concentrations of the initial solution and for different ratios of isomers in the benzene hexachloride being treated.

When the organic solution of benzene hexachloride to be treated is the crude reaction mixture in which the hexachloride was made, the solution may contain some dissolved unreacted chlorine. A suitable amount of a chlorine acceptorf' such as sodium hydroxide, sodium carbonate, or sodium bicarbonate, or of a reducing agent such as sodium sulfite, may be added to the water in the steam still to neutralize such chlorine without interfering with the purification and separation procedure. Such simple precaution prevents corrosion of the equipment and possible contamination of the product with metal chloride.

The followin table illustrates typical results obtained in the method of the present invention when various amounts of the originally dissolved benzene hexachloride have been precipitated in the partial steam distillation of solvent from the solution. The data presented are based speciflcally on a treatment applied to a 15 per cent solution by weight of benzene hexachloride in a solvent consisting of about parts by weight of benzene and 16 parts by weight of propylene chloride. The benzene hexachloride had an analysis of about 68 per cent alpha, 11 per cent beta, 13 per cent gamma, and 8 per cent delta isomers. 7

Table Per cent of original benzene hexachloride precipitated as solids 40 Per cent of benzene hexachloride left in solution Per cent of original solvent distilled 1.

Concentration of benzene hexachloride in uncrystallized solution, per cent Per cent of original gamma isomer left in solution 91. 5

Gamma isomer content 01 precipitated solids, per cent 2. 8

. Gamma isomer content of benzene hexachloride recoverable from liquid phase, per cent l9 8 It is noted that, when only 40 per cent of the benzene hexachloride has been precipitated by steam distillation, while 91.5 per cent of the orig- Gil inal gamma isomer remains in the liquid phase, the crystals obtained from that phase are only half again as rich in gamma isomer as the originalmixture. When, however, 60 per cent of the benzene hexachloride is precipitated and separated from the rest, the gamma content of the benzene hexachloride in the liquid phase is double the amount in the original mixture. When precipitation is carried to 80 per cent before separation, the liquid phase yields a benzene hexachloride with 2.5 times the original concentration of gamma isomer. However, due to the great difference in weight between the precipitated material and the material which is left dissolved, at about 82 per cent precipitation the two fractions contain about equal weights of gamma isomer, and preliminary precipitation of more than 80 but less than 100 per cent of the benzene hexachloride results in most of the gamma isomer remaining in the precipitated fraction even though the liquid fraction has a much higher percentage of gamma isomer in its much smaller mass.

The liquid portion of the benzene hexachloride which passes through the filter on which the precipitated crystals are caught has been referred to as a solution. It is noted from the foregoing table that the amount of solvent left in that layer is small and that precipitation of from 40 to 80 per cent of the original benzene hexachloride leaves an oil phase having a concentration correspondingly from 43 to 91 per cent, in the example given. In the runs on which the table is based, the amount of solvent which has been steam distilled from the original solution to arrive at these concentrations ranges from 86 to 99.5 per cent of that used.

In a specific example, the method of the present invention was applied to 500 milliliters of the crude reaction product obtained by adding chlorine to benzene in the presence of propylene as a catalyst, as described by K. C. Kauer, F. N. Alquist and E. C. Britton in copending application Serial No. 695,238, filed September 6, 1946.

' The solution weighed 482.5 grams and contained 76.5 grams, or 15.8-per cent of benzene hexachloride dissolved in 406 grams of a mixture of about 90 per cent benzene and 10 per cent propylene chloride.- The dissolved benzene hexachloride contained,

Per cent Alpha-isomer 58 Beta-isomer 9 Gamma-isomer 14 Delta-isomer 15 Epsilon-isomer 4 was filtered to separate the precipitated crystals both from the water and from a waterimmiscible liquid having an oily appearance and the odor oi the benzene-propylene chloride mixture used as a solvent. The crystals deposited on the filter, when dried, weighed 54 grams ('71 per cent of the charge) and contained,

Per cent Alpha-isomer 44 Beta-isomer Gamma-isomer 29 Delta-isomer 22 Epsilon-isomer The total amount of gamma isomer present in the original charge was 10.7 grams, based on infra-red analysis of a representative sample. On the same basis. the first crystals from the steaming operation contained 3.8 grams of the gamma isomer and the second crystals, from the water-immiscible oily layer, contained 6.5 grams of the gamma isomer. The apparent 4 per cent discrepancy between the 10.7 grams charged and 10.3 grams recovered is due to losses in handling or to experimental error in the infrared analysis, or to both. The fraction having the high gamma isomer content was a white crystalline mass with none of the irritating odor usually associated with commercial benzene hexachloride.

In a similar manner, a batch of 665 pounds of a solution 01' benzene hexachloride in benzene, containing 100 pounds of the solute of which about 12.5 per cent was the gamma isomer, was steam distilled until about 60 per cent of the benzene hexachloride had separated as crystals suspended in the water present in the still. The mass was cooled to 40 C. and filtered. The recovered crystals were dried, weighed and analyzed. They represented 61 per cent oi. the original charge and contained 3 per cent 01' the gamma isomer of benzene hexachloride. The immiscible layer in the filtrate was steamed further to remove residual solvent and yielded, on cooling, about 39 per cent 01' the original charge as crystals having a gamma isomer content of 2'7 per cent. Thus, oi! the gamma isomer originally present, 1.8 poimds separated with the first crop of crystals and 10.5 pounds, or about 80 per cent, was found in the second crop.

The method, which has been described as a batch process, is capable of being operated continuously, using known types of continuous steam strippers coupled with continuous filters to separate the crystals first appearing from the layer of oily solution rich in the gamma isomer. Similarly, though operation at atmospheric pressure is usually preferred, the steam distillation may be effected at reduced pressure if desired, to minimize the temperature at which the solvent present in the original solution may be distilled with steam.

I claim:

1. The method which comprises subjecting to steam distillation in the presence of water, the solution of a mixture of isomeric forms of henzene hexachloride in an organic solvent consisting principally of benzene and resulting from the addition chlorination of benzene to the point at which the liquid medium first becomes substantially saturated with benzene hexachloride, continuing the steam distillation until from about 40 to per cent of the benzene hexachloride originally present is precipitated from the solution; filtering the mixture and separating the precipitated crystals, characterized by having a reduced content of the gamma isomer as compared with the charge, from the water and the remaining oily solution of benzene hexachloride; removing the solvent from the remaining oily solution; and cooling the residue to crystallize the remaining benzene hexachloride, characterized by containing most of the gamma isomer originally present in the charge.

2. The method which comprises subjecting to steam distillation in the presence of water, the solution of a mixture of isomeric forms of benzene hexachloride in an organic solvent consisting principally of benzene and resulting from the addition chlorination of benzene to the point at which the liquid medium first becomes substantially saturated with benzene hexachloride, continuing the steam distillation until at least 40 per cent of the benzene hexachloride originally present is precipitated from solution; discontinuing the steam-distillation before more than 99.5 per cent of the original solvent has been steam distilled; filtering the mixture and separating the precipitated crystals, characterized by having a reduced content of the gamma isomer as compared with the charge, from the water and the remaining oily solution of benzene hexachloride; removing the solvent i'rom the'remaining oily solution; and cooling the residue to crystallize the remaining benzene hexachloride, characterized by containing most of the gamma isomer originally present in the charge.

JAMES M. SCOVIC.

REFERENCES CITED The following references are of record in the file of this patent:

FOREIGN PATENTS vol. 45, pages 231-235 (1912). 

1. THE METHOD WHICH COMPRISES SUBJECTING TO STEAM DISTILLATION IN THE PRESENCE OF WATER, THE SOLUTION OF A MIXTURE OF ISOMERIC FORMS OF BENZENE HEXACHLORIDE IN AN ORGANIC SOLVENT CONSISTING PRINCIPALLY OF BENZENE AND RESULTING FROM THE ADDITION CHLORINATION OF BENZENE TO THE POINT AT WHICH THE LIQUID MEDIUM FIRST BECOMES SUBSTANTIALLY SATURATED WITH BENZENE HEXACHLORIDE, CONTINUING THE STEAM DISTILLATION UNTIL FROM ABOUT 40 TO 80 PER CENT OF THE BENZENE HEXACHLORIDE ORIGINALLY PRESENT IS PRECIPITATED FROM THE SOLUTION; FILTERING THE MIXTURE AND SEPARATING THE PRECIPITATED CRYSTALS, CHARACTERIZED BY HAVING A REDUCED CONTENT OF THE GAMMA ISOMER AS COMPARED WITH THE CHARGE, FROM THE WATER AND THE REMAINING OILY SOLUTION OF BENZENE HEXACHLORIDE; REMOVING THE SOLVENT FROM THE REMAINING OILY SOLUTION; AND COOLING THE RESIDUE TO CRYSTALLIZE THE REMAINING BENZENE HEXACHLORIDE, CHARACTERIZED BY CONTAINING MOST OF THE GAMMA ISOMER ORIGINALLY PRESENT IN THE CHARGE. 